Abstract
A model of the mitotic cycle, with independent probability density functions for the durations of the phases G1, S, and G2, is examined. A Monte Carlo program for the computation of percentage labeled mitoses in accordance with the model is described and results are shown for log-normal distributions of the phase durations. This work is applied to the results of an experiment performed with transplanted Marshall rat mammary tumors. From the cell cycle times and age distributions thus obtained, the doubling time and labeling index are calculated and related to observed values.